Carrie Underwood's toughest audience is living in her belly. The pregnant singer, 31, told Jimmy Fallon on Monday, Dec. 8, that she performs for her yet-to-be-born son, and feels pressure to make it "good."

"I feel like he kind of feels… he hears me singing whether he would like that or not," Underwood said on The Tonight Show. "I mean, that's what I do, he doesn't have a choice."

"I do feel like I have to sing good all the time," the "Something in the Water" performer continued. "Like, if I'm just in my car driving, I feel like I have to make it good because he's listening. He's always in there."

Underwood is expecting her first child with husband Mike Fisher. The pair, who wed in July 2010 after getting engaged in December 2009, announced their happy news this past September. The country crooner isn't the only one taking the stage in the couple's Nashville home, though; hockey player Fisher, 34, also exercises his pipes around his talented wife.

"It's always surprising," the American Idol alum told Fallon of Fisher. "He doesn't actually know the real lyrics to any song, except for like, 'Whoomp! (There It Is)' will come on the radio and he knows every word to that. Yet, I'll ask him what my current single is and he sings all the wrong words. He has no clue."